This invention relates to a high intensity discharge (HID) lamp ballast and, more particularly, to an improved driving scheme for an HID ballast down converter.
An HID lamp generally includes high pressure mercury, high pressure sodium, metal halide, high pressure metal vapor and low pressure sodium lamps. The alternating current supplied to each of these lamps is provided through commutation of a D.C. current. The D.C. current is provided from a down converter which serves as a D.C. current source, the D.C. current being converted into a D.C. square wave by a commutator.
The down converter includes a switch and a choke. The time duration during which the switch is closed (i.e. turned ON) controls the amount of energy stored within the choke. The amount of the power produced by the down converter is supplied to the lamp based on the time duration during which the switch is turned ON as compared to the time duration during which the switch is turned OFF (e.g. switching frequency and duty cycle).
The down converter switch, typically a MOSFET, is generally turned ON at a frequency of between about 25 to 80 KHz. The MOSFET transition time (i.e. time during which the switch changes from the ON to OFF state or from the OFF to ON state) is about 100 to 500 nanoseconds. During each of these transitions, the voltage across the MOSFET typically changes several hundred volts (e.g. about 300 volts). The MOSFET is turned ON and will remain turned ON by providing a voltage of, for example, approximately 15 volts between the gate and source of the MOSFET. To turn the MOSFET OFF and retain the MOSFET in an OFF state, the voltage between the gate and source of the MOSFET must be, for example, about 0 volts. The voltage at the source of the MOSFET can vary between 0 volts and several hundred volts (e.g. 300 volts). The gate voltage must therefore vary from about 315 volts (i.e. to turn the MOSFET ON when the voltage at the source is about 300 volts) to about 0 volts (i.e. to turn the MOSFET OFF when the voltage at the source is about 0 volts).
When turning the MOSFET from its ON state to its OFF state, the voltage at the gate of the MOSFET must first be adjusted from approximately 315 volts to approximately 300 volts to create a voltage difference of approximately 0 volts between the gate and source. As the voltage of the source rapidly decreases to approximately 0 volts, the gate voltage must rapidly decrease to 0 volts to maintain the MOSFET in its OFF state (i.e. maintaining the voltage between gate and source at approximately 0 volts). The gate voltage therefore follows the source voltage in maintaining the MOSFET in its OFF state. Similarly, in turning the MOSFET from its OFF state to its ON state, the MOSFET gate voltage must be maintained, for example, at approximately 15 volts above the voltage at the MOSFET source. Therefore, the voltage at the gate must rapidly rise from approximately 15 volts to approximately 315 volts. As can readily be appreciated, such wide variations of voltage during such short transitions (i.e. about 100 to 500 nanoseconds) can be difficult to achieve.
In adjusting the voltage applied to the gate of the MOSFET between several hundred volts and approximately 0 volts, conventional driving circuitry employs a level shifter. The level shifter shifts the driving voltage produced by a current mode controller of the down converter relative to the voltage at the MOSFET source. The output from the level shifter, which is applied to the MOSFET gate, is subject to distortion from electromagnetic interference (EMI) and/or parasitic capacitances within the MOSFET and/or level shifter. Such distortion makes it difficult to apply and/or maintain the desired voltage at the gate relative to the source of the MOSFET. Consequently, the MOSFET can be turned ON when it should be turned OFF and turned OFF when it should be turned ON.
The lever shifter, by being subjected to an extremely fast switching transition, high voltage levels and distortion of its output (driving) signal, represents the weak link in a conventional down converter driving scheme. This drawback is based on the driving signal applied to the gate of the MOSFET being derived from two different voltage references. These references, at times, are separated from one another by relatively large differences in voltage over very short periods of time (i.e. very fast switching transitions).
A conventional driving scheme, such as disclosed in Great Britain Patent No. 1,053,236, eliminates the need for a level shifter by connecting one end of a lamp to the positive terminal of the power source with the switch connected between the other end of the lamp and the negative terminal of the power source. When one end of an HID lamp, however, is continuously coupled to the positive terminal of the D.C. power source (positive terminal of the down converter), sodium ions within certain types of HID lamps begin to migrate out of the lamp thereby reducing lamp life. It is therefore highly desirable to avoid continuously coupling either terminal of the lamp to the positive terminal of the power supply.
Accordingly, it is desirable to provide an HID ballast down converter having an improved driving scheme which requires only one reference voltage level. In particular, the HID ballast down converter should employ a driving scheme which eliminates the need for a level shifter in controlling the conductive and non-conductive states of the down converter switch. The HID ballast down converter driving scheme should also minimize ion migration out of the lamp.